Fastener retention system

ABSTRACT

A vertebral plate system and associated method minimizes the number of required multiple configurations and sizes of hardware on hand during surgery for repair, stabilization or fusion of bone segments. A novel aspect of the invention is a resilient split ring that cooperates with angled contact surfaces in a plate for allowing convenient and fast dynamic locking between the plate and a bone screw. In another aspect of the present invention, a screw or similarly elongated fastener having a generally spherical or part-spherical head with a section of helical threads is configured in a generally horizontal band and positioned around a maximum-diameter section of the head. The screw is loosely retained to a first structure by a snap-ring formed from a ring of elastic material having a slot cut so that the ring is a “C” ring. The ring is seated on an annular shoulder formed in a through-hole in the first structure. The section of the through-hole above the shoulder tapers inwardly in an upward direction to a minimum diameter that is smaller than the outside diameter of the ring when the ring is in an unstressed state. The inner diameter of the ring has an edge that is adapted to cooperate with and ride in the threads on the screw head.

RELATED APPLICATIONS

[0001] This application is related to, and claims priority from, thefollowing earlier-filed U.S. Provisional Patent Applications: (SerialNos.) 60/422,978 (filed Nov. 4, 2002); 60/479,797 (filed Jun. 20, 2003);and 60/476,642 (filed Jun. 9, 2003). Each is incorporated herein byreference.

TECHNICAL FIELD

[0002] The present invention relates to orthopedic implantable devices,related systems, and associated methods of use and, more particularly,to a plate system and associated method for joining two or more bonesegments, such as vertebrae.

BACKGROUND OF THE INVENTION

[0003] In orthopedics it is known to use various types of platesfastened to one or more bone segments in order to join said bonesegments in a predetermined relationship for stabilization and/orcontrolled movement. Such stabilization or controlled movement isdesirable when repairing bone segments that have become deteriorated,damaged or degenerative, such as due to trauma or disease. Suchstabilization or controlled movement may be used in cooperation with oneor more fusion or stabilizing devices, or may be used alone.

[0004] The use of such plates or systems is employed in the treatment ofvertebral bodies. It is known, for example, to use plating systems tojoint one or more adjacent vertebrae for stabilization or to enhancefusion.

[0005] Various known plates used in the above-described manner are knownto have shortcomings. Such shortcomings include lack of versatility soas to require multiple configurations and sizes of hardware on handduring surgery; lack of anatomical correspondence with resultant poorfit, high stress concentrations and unnatural load forces on adjacent orfused bone segments; susceptibility to pulling out due to limitedarrangement of bone fastener or bone screw angles; and othershortcomings inherent to known designs.

OBJECTS AND SUMMARY OF THE INVENTION

[0006] It is an object of the present invention to provide a platesystem and associated method that minimizes the number of requiredmultiple configurations and sides of hardware on hand during surgery forrepair, stabilization or fusion of bone segments.

[0007] It is an object of the present invention to provide a platesystem that allows versatility and post-installation adjustment betweenfully dynamized, angularly adjustable and fixed bone screws.

[0008] It is an object of the present invention to provide a platesystem and associated method that have optimal anatomical correspondencewith bone segments to be joined to optimize fit and to minimizeundesirable stress concentrations and unnatural load forces on adjacentor fused bone segments.

[0009] It is an object of the present invention to provide a platesystem and associated method that minimizes susceptibility to pullingout due to limited arrangement of bone fastener or bone screw angles.

[0010] It is an object of the present invention to provide a fastenerretention assembly that loosely retains a fastener with respect to onestructure in a manner such that, when placed into engagement withanother structure, the fastener can be easily, reliably and preciselydriven into the second structure.

[0011] It is an object of the present invention to provide a fastenerretention assembly that is superior to know fastener retentionassemblies in applications where limitations or constraints in terms ofspace, precision requirements, durability, material compatibility, easeof use, safety, and other factors exist.

[0012] It is an object of the present invention to provide a fastenerretention assembly that is suitable for superior performance inapplications specifically directed to joining an orthopedic implant to abone.

[0013] It is an object of the present invention to provide a fastenerretention assembly that is suitable for superior performance inapplications specifically directed to retaining a bone screw to acervical plate before, during and after the bone screw is driven into abone to which the plate is fastened.

[0014] These and other objects are described below or inherent withrespect to the present invention.

[0015] A vertebral plate system and associated method according to apreferred embodiment of the present invention minimizes the number ofrequired multiple configurations and sizes of hardware on hand duringsurgery for repair, stabilization or fusion of bone segments. A novelaspect of the invention is a resilient split ring that cooperates withangled contact surfaces in a plate for allowing convenient and fastdynamic locking between the plate and a bone screw.

[0016] In another aspect of the present invention, the present inventionis directed to a screw or similarly elongated fastener having agenerally spherical or part-spherical head with a section of helicalthreads being configured in a generally horizontal band and positionedaround a maximum-diameter section of the head. The screw is looselyretained to a first structure by a snap-ring formed from a ring ofelastic material having a slot cut so that the ring is a “C” ring. Thering is seated on an annular shoulder formed in a through-hole in thefirst structure. The section of the through-hole above the shouldertapers inwardly in an upward direction to a minimum diameter that issmaller than the outside diameter of the ring when the ring is in anun-stressed state. The inner diameter of the ring has an edge that isadapted to cooperate with and ride in the threads on the screw head.

[0017] In operation, the ring is flexed or stressed in a manner to pushit through the minimum diameter section at the top of the through holeof the first structure. After the ring passes down the through-hole itrests upon the shoulder and is expanded to its unstressed state. Theshaft section of the screw is advanced through the ring and throughholeuntil the screw head begins to pass through the inner diameter of thering. As the spherical shape of the screw head passes through the ring,the maximum diameter section with threads approaches the ring. The ringthen gets slightly expanded as the maximum diameter section approachesthe ring edge. At this point, further advancement of the screw causesthe edge of the ring to engage the threads on the screw head. While thethreads are engaged by the ring, advancement of the ring requiresrotation in correspondence with the threads. This allows the screw topass through the ring in a manner requiring much less axial force thanif no threads existed, since the profile of the spherical head of thescrew would require the ring to expand significantly more. After themaximum diameter section of the head is past the ring and furtheradvancement of the screw past the ring continues, the screw reaches apoint in which the ring is past the threaded section and is engaging thesmooth, spherical portion of the screw head above the threaded band. Thereturn force of the ring that biases it toward its un-stressed positioncauses the ring to slide up on the spherical head of the screw until itreaches an un-stressed condition, at which point the screw head cannotbe backed out of the ring or the through-hole. This is because if thescrew is merely pushed upward, the ring will be pushed up against theminimum diameter area of the tapered through hole and constrainedagainst expansion adequate to let the screw head pass.

[0018] In order to release the screw and allow it to be removed from thering and through-hole, a narrow instrument must be inserted to pushdownwardly on the ring, causing it to expand slightly around the screwhead until the inside edge engages the screw thread, at which timecounter-rotation will advance the screw head through and out of the ringand through-hole in a manner opposite of its insertion.

[0019] This assembly is adapted for use when joining the screw and thefirst structure to a second structure. For example, the screw can be abone screw for joining a first structure, such as a cervical plate, to asecond structure such as a vertebrae. In such a situation. The screwwill have a threaded shaft adapted to be driven into a hole in saidvertebrae.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020]FIG. 1 is a schematic, anterior view of the preferred embodimentof the present invention plate system.

[0021]FIG. 2 is a partial, anterior view of the preferred embodiment ofthe present invention plate system.

[0022]FIG. 3 is a top view of the preferred embodiment of the presentinvention plate system.

[0023]FIG. 4 is a partially exploded perspective view of the preferredembodiment of the present invention plate system.

[0024]FIG. 5 is a front view of a first embodiment bone screw fasteneraccording-to the present invention plate system.

[0025] FIGS. 6(a)-6(c) are perspective views of various alternative bonescrew sleeve arrangements according to the present invention.

[0026]FIG. 7 is a front, cross-sectional view of a second embodimentbone screw fastener according to the present invention plate system.

[0027]FIG. 8 is a partial, side view of the preferred embodiment of thepresent invention plate system.

[0028]FIG. 9 is a perspective view of a second embodiment of the presentinvention plate system.

[0029]FIG. 10 is a partial, cross-sectional view of a third embodimentof the present invention plate system directed to a locking system forretaining a plate to bone structure.

[0030]FIG. 11 is a side view of a bone screw according to the thirdembodiment of the present invention.

[0031]FIG. 12 is a front and side view of a locking ring according tothe third embodiment of the present invention.

[0032]FIG. 13 is a side, cross-sectional view of the locking ringaccording to FIG. 12.

[0033]FIG. 14 is a partial, side cross-sectional view of a plate withbone screw and locking ring assembled according to the third embodimentof the present invention.

[0034]FIG. 15 is a partial, side cross-sectional view of a plate withbone screw and locking ring assembled and locked according to the thirdembodiment of the present invention.

[0035]FIG. 16 is a cross-sectional view of a bone screw according to thethird embodiment of the present invention.

[0036]FIG. 17 is a side view of an insertion instrument for use with abone screw and locking ring according to the third embodiment of thepresent invention.

[0037]FIG. 18 is a partial side view of the insertion instrument of FIG.17.

[0038]FIG. 19 is a partial side view of the insertion instrument, plate,bone screw and locking ring according, to the third embodiment of thepresent invention.

[0039]FIG. 20 is a front view of a screw according to the presentinvention assembly.

[0040]FIG. 21 is a perspective view of a bone plate according to thepresent invention assembly.

[0041]FIG. 22 is a side, cross-sectional view of a retention ring and apartial view of the plate according to the present invention.

[0042]FIG. 23 is a side, cross-sectional view of a retention ring and apartial view of the plate according to the present invention, in whichthe screw of FIG. 20 is partially advance through the ring.

[0043]FIG. 24 is a side, cross-sectional view of a retention ring and apartial view of the plate according to the present invention, in whichthe screw of FIG. 20 is fully advanced through the ring.

[0044]FIG. 25 is a top view of the ring component according tothe-present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0045] The present invention system and method may be used to join avariety of bone segments for stabilization, controlled or restrictedmovement, and/or fusion. The present invention is particularlywell-suited for use in joining vertebral bodies and, thus, is presentedfor use in joining cervical vertebrae in the description of thepreferred embodiment. This is not intended, however, to be limiting onthe scope of the present invention.

[0046] Referring to FIG. 1, there is shown schematically a group ofcervical vertebral bodies (10) comprising two adjacent vertebrae (12,14) having a disc space (16) therebetween, viewed from a side view ormedial perspective. A plate (18) according to the present invention isshown fastened to the anterior side (20) of the column of vertebrae in amanner bridging across the disc space (16). Bone screws (22, 24) areused to fasten the plate in position in accordance with the presentinvention to stabilize the vertebrae (12, 14) for controlled movement orfusion.

[0047] The plate (18), which may be made from titanium or a suitablemetal or non-metal, as shown in FIG. 2, comprises first and second ends(26,28) each having a pair of fastener slots (30, 32, 34, 36); and amiddle portion (38) having a central slot (40). At the first and secondends (26,28), respectively, are a recessed area (42), a lock screw hole(44), a probe insertion hole (46), and a further recessed apex (48).

[0048] As shown in the top view of FIG. 3, the cross-sectional shape ofthe plate (18) is such that the radius (50) of the outer surface (52) issmaller than the radius (54) of the inner surface (56) which contactsthe vertebrae (12, 14). This feature facilitates optimal fit andpositioning of bone screws or fasteners (72) such that they are angledinward toward each other. Preferably, the, fasteners (72) are angled atabout 6 degrees with respect to a direction normal to a tangent at themidpoint of the inner surface (56). This arrangement inherently reducespullout susceptibility.

[0049] Referring to FIG. 4, a sleeve (58) is positioned in each fastenerslot (30, 32, 34, 36). Each sleeve (58) is retained in the plate (18) bya sleeve cover (60) positioned at each end (26, 28) of the plate (18).Each sleeve cover (60) fits into one of the recessed areas (42),respectively, and is fixed to the plate (18) by a lock screw (62)passing through a central hole (64) in the sleeve cover (60) which isaligned with the lock screw hole (44) for threadedly receiving the lockscrew (62) therein. The profile of each sleeve cover (60) overlaps thetops of the respective sleeves (58) to prevent them from backing up andout of the plate (18). Each sleeve (58) includes a bone fastener hole(66) having threads (68) therein to retain the head (70) of a bonefastener (72) in the plate (18). Tightening each threaded head (70)causes the sleeves (58) to expand into locking engagement with the plate(18). Alternatively, as shown in FIG. 7, a snap ring (76) andcooperating channel (78) may be used in lieu of a threaded headconnection.

[0050] As shown in FIG. 6(a), each sleeve (58) is round in top view and,thus, slideable within each groove (30). In addition, as shown in FIG.7, each sleeve (58) has a semispherical lower portion (74) adapted tosit in a similarly shaped cup or seat (80) to allow relative pivoting ofthe sleeve (58) and fastener (72) with respect to the plate (18).Pivoting is preferably about 10 degrees from the axis, as shown in FIG.8. This mode of enabling sliding movement and angular adjustment of thefastener (72) relative to the plate (18) is referred to as a “fullydynamized” mode. In this fully dynamized mode, the permitted slidingminimizes friction wear. In the case of fusion applications, a graft maybe provided with in-line dynamic compression as the grooves (30, 32, 34,36) are tangent to the lordotic curve of the plate (18).

[0051] Referring to FIG. 6(b), a crescent shaped insert (82) may bepositioned in one or more of the grooves (30) to constrain the sleeve(58) from translation within the groove while allowing pivoting of thesleeve (58) and fastener (72) with respect to the plate (18). Thecrescent insert (82) has a concave end (84) and a convex end (86). Theconcave end (84) forms a semi-spherical cup section for enabling thesleeve (58) and fastener (72) to pivot relative to the plate (18). Thismode allows variable angle adjustment of the bone fastener (72) whileprohibiting sliding movement.

[0052] Another alternative for mounting the fastener (72) to the plate(18) is a fixed angle mode in which the bone fastener (72) neitherslides nor rotates, as shown in FIG. 6(c). This is accomplished by usinga modified sleeve (88) that is shaped and sized to fill the entiregroove (30). The modified sleeve (88) cannot slide within the groove(30) and it has a threaded opening (90) adapted to fixedly attach thefastener (72) thereto.

[0053] Referring to FIG. 4, each sleeve cover (60) has a probe insertionhole (92) that aligns with the probe insertion hole (46) of the plate(18) when the sleeve cover (60) is positioned thereon. The two probeinsertion holes (46, 92), when aligned, allow a probe or otherinstrument (not shown) to be positioned therein to pin or hold the plate(18) against the bone or vertebrae to which the plate (18) will befastened during installation. Each sleeve cover (60) also includes anapex cutout (94) that, when aligned with the apex recess (48) of theplate (18), provides clearance for the esophagus of a patient in whichthe plate (18) is installed in the cervical region.

[0054] While the use of the sleeve covers (60) constrains each fastener(72) and sleeve (58) while allowing pivoting, as shown in FIG. 8,another significant advantage is the ability to remove the sleeve covers(60) post-installation to adjust or change to or from any one of thefasten modes of fully dynamized, variable angle, and fixed angle.

[0055] In use, the plate (18) is positioned against the vertebrae (12,14) to be joined and held in place while a plurality of fasteners (72)are inserted through the grooves (30, 32, 34, 36) and driven into thevertebrae (12, 14). Each fastener (72) is mated with one of the sleeve(58) or modified sleeve (88). The sleeve covers (60) are fastened to theplate (18) using lock screws (62) so that the sleeve covers (60) overlapand prevent backing out by the fasteners (72).

[0056] Alternatively, as shown in FIG. 9, a plate (100) according toanother embodiment of the present invention has three sets of grooves(102) adapted to receive a total of six bone fasteners (not shown) andadapted to span and connect three vertebrae. The plate (100) is similarto the above-described first embodiment in other respects.

[0057] Another embodiment of the present invention directed to a systemfor locking a vertebral plate is described with respect to FIGS. 10-19.The locking system described in this embodiment is intended for use withany one of a variety of plate systems and configurations including, butnot limited to, the specific plates and systems described herein.Referring to FIG. 10, a vertebral plate (203) includes at least one hole(204) having a conical section (205) and a hemispherical section (206).A tapered lock ring (208) has a cut (210) so that it forms a resilient“C” shaped ring. Referring to the cross-sectional view of FIG. 13, thering (208) has an outside diameter angle βand an inside diameter angleα. In the preferred embodiment angle β is greater than angle α, but thismay be varied according to desired performance, material properties, andsurrounding geometric parameters. Initially, the lock ring (208) isflexed to pass it though the opening (212) of the hole (204) until itre-expands and rests on a shoulder (214) that separates the conicalsection (205) from the hemispherical section (206), as shown in FIG. 10.

[0058] A bone screw (214) having a generally spherical or partiallyspherical head (216) and a bone-engaging shaft (218) such as a threadedshaft is passed through the hole (204) and ring (208). The maximumdiameter section of the screw head (216) is greater than the innerdiameter of the lock ring (208) such that, when pushed through the lockring (208), the screw head (216) expands the ring (208) as shown in FIG.14. Once the screw head maximum diameter section is past the ring (208),the ring (208) rises upwardly along the contours of the screw head (216)under its own kinetic energy that causes it to return to its originalshape, as shown in FIG. 15. In this position, the conical section (205)of the hole (204) mates with the exterior, angled surface (220) of thering (208).

[0059] Torque and rotational movement may be applied to the bone screw(214) by a driver instrument (222). The driver (222) comprises a shaft(224), a handle (226), and a screw-engaging head (228). The head (228)may have flats (230) arid a snap ring (232). The flats (230) cooperatewith flats (234) and a ring groove (236) in an opening (238) in thescrew head (216). Such torque and rotational movement are applied to thebone screw (214), which is typically threaded, during insertion orremoval.

[0060] During removal, it is necessary to expand and re-position thelock ring (208) downwardly in order to allow passage of the maximumdiameter section of the screw head (216) through the lock ring (208).This is achieved by implementing a thin, sleeve end (240) of cylindricalsegments in a manner in which it pushes the lock ring (208) downwardlywithin the hole (204) as shown in FIG. 19. When the ring (208) is pusheddownwardly over the screw head (216) it is expanded and held in thisposition by the sleeve end (204) while the screw (214) is rotated andremoved by backing out. After the screw head maximum diameter section isbacked out past the ring (208), the ring (208) can be released and thescrew (214) removed entirely.

[0061] According to another alternative embodiment, the sleeves could beassembled in the plate by chilling the sleeve and heating the plate tocause corresponding respective contraction and expansion, such that whenthe respective temperatures return to ambient conditions the fasteners,sleeves and plate are relatively locked to each other.

[0062] The present invention can be used with or without fusion elementsincluding fusion inserts and fillers such as bone pastes, bone chips andbone morphogenic proteins (BMP). It may also be used by itself or incombination with other devices and it may be used in the cervical spinalapplications or other types including lumbar.

[0063] Another embodiment of the present invention has variousapplication in which a first structure retains a fastener before, duringand after being joined to a second structure by the fastener. While thepreferred embodiment is directed to a bone screw for joining a cervicalplate vertebral bone, the embodiment is by way of example and is notintended to limit the scope of the invention.

[0064] As shown in FIG. 20, a bone screw (310) has a shaft portion (312)which can be threaded, a generally spherical head section (314), and athreaded section (316) of helical threads spanning the maximumhorizontal diameter. A conventional driver engagement configuration (notshown) on the top of the head (314) such as a slot, a hexagonal hole, orthe like is provided to enable rotational force to be applied to thescrew (310) for rotationally driving the screw (310). The screw (310)may be made of titanium, stainless steel, or other suitable materials.The head (314) may be a fill sphere, a part-sphere, or a similarconfiguration in which a maximum horizontal diameter occurs betweenupper and lower axial sections of the head.

[0065] Referring to FIG. 21, a cervical plate (318) of the typegenerally known for joining two adjacent vertebra in a stabilizingmanner is provided with a plurality of screw holes (320), each adaptedto receive the ring component (332) and screw (310) assembly accordingto the present invention.

[0066] As shown in FIG. 22, each hole (320) in the plate (318) has anupper minimum diameter section (324), a tapered section (326) thattapers outwardly in a generally increasing diameter manner downwardlyfrom the upper minimum diameter section (324), a shoulder (328), and agenerally tapered seat section (330). A retention ring (332), also shownin top view in FIG. 25, has an inner diameter edge (334), an innerdiameter upwardly tapering wall (336), an outer diameter upwardlytapering wall (338), and a slot (340). The retention ring (332) is madefrom a resilient material that allows it to be stressed or compressed ina manner that allows it to be pressed past the minimum diameter section(324) and into the hole (320) until it sits on the shoulder (328). Whileseated on the shoulder (328), the ring (332) has expanded back to itsun-stressed shape since the inner diameter of the tapered section (326)near the shoulder (328) is greater than the outside diameter of the ring(332).

[0067] The screw (310) is placed through the ring (332) and hole (320)and advanced, for example into a vertebral bone, until the head (314)engages the ring (332). As the maximum horizontal diameter of the head(314) moves toward the edge (334) the ring (332) slightly expands untilthe edge (334) enters the threads (316), as shown in FIG. 24. At thispoint, the screw (310) is axially advanced further by rotation so thethreads (316) ride along the edge (334). As the edge (334) moves pastthe threaded section (316), it engages the smooth, spherical head of thescrew (310) above the threads (316), as shown in FIG. 25.

[0068] The ring (310) preferably has a tapered inside wall (336) and atapered outside wall (338). The hole (320) preferably has a taperedsection (326). This combination enables the ring (310) to be wedgedbetween the screw (310) and the plate (318) by the upward force of thescrew head (310) which imparts both vertical and horizontal forces tothe ring's inner wall (336) which, in turn, are transmitted to thehole's inner wall (326) via the outer wall (338). If upward force of thescrew (310) is increased, the ring (310) is wedged tighter. Because thering cannot be substantially compressed, and because it is appropriatelysized with respect to minimum diameter (324), it is prevents the screwhead (314) from backing out. The screw head (314) is supported andprevented from slipping out of the hole (320) on the bottom side by aseat (330). The seat (330) may be tapered, conical of hemi-spherical.

[0069] In order to release the screw (310) and allow it to be removedfrom the ring (332) and through-hole (320), a narrow instrument (notshown) must be inserted to push downwardly on the ring (332), causing itto expand slightly around the screw head (314) until the inside edge(334) engages the screw thread (314), at which time counter-rotationwill advance the screw head (314) through and out of the ring (332) andthrough-hole (320) in a manner opposite of its insertion.

[0070] While the present invention has been described herein, variousmodification may be made without departing from the scope of theinvention.

What is claimed is: 1) A fastener retention system comprising a fastenerhaving a first end and a second end, said first end being adapted tofixedly engage a first body, and said second end being adapted to engagea second body; and a resilient retention member cooperating with saidsecond body and said second end of said fastener such that said fastenermay be selectively changed between a non-retained condition and aretained condition, with respect to said second body, when saidretention member is changed between a relaxed state and a flexed state,in order to join said first body and said second body. 2) A retentionsystem according to claim 1 wherein said first end of said fastenercomprises screw threads. 3) A retention system according to claim 1wherein said first body comprises bone; and said second body comprises asurgically implantable device. 4) A retention system according to claim3 wherein said fastener is a bone screw. 5) A retention system accordingto claim 3 wherein said surgically implantable device is adapted to spanbetween at least two bone structures. 6) A retention system according toclaim 5 wherein said surgically implantable device is a plate that isfastened to each of said at least two bone structures. 7) A retentionsystem according to claim 1 wherein said retention member is a splitring. 8) A retention system according to claim 7 wherein said split ringhas a first diameter when it is in a relaxed state, and is adapted toresiliently expand to larger diameters when it is in a flexed state;said second end of said fastener comprises a locking portion of saidfastener having a large diameter section greater than said firstdiameter and positioned between two smaller diameter sections; and saidsplit ring is positioned within said second body such that said fasteneris retained in said second body when said split ring is positionedaround one of said smaller diameter sections in a manner in which saidlarge diameter section must be passed through said split ring in orderto remove said fastener. 9) A retention system according to claim 8,wherein said second end of said fastener is threaded. 10) A retentionsystem according to claim 8, wherein said second end of said fastener isgenerally spherical and is threaded over a portion including said largediameter section. 11) A retention system according to claim 8, whereinsaid second body comprises an opening in which said split ring ispositioned, said opening having a surface diameter at the outer surfaceof said second body and a subsurface diameter beneath said outer surfacethat is greater than said surface diameter. 12) A retention systemaccording to claim 11, wherein said split ring has an outer diameter inits relaxed state that is greater than said surface diameter. 13) Asystem for joining a first body to a second body, said system comprisinga fastener comprising an elongated shaft of varying diameter along alongitudinal axis; and a resilient member adapted to be selectivelypositioned around said fastener and sized such that said resilientmember is in an un-stressed state at various positions along saidlongitudinal axis, and said resilient member is in a stressed state inat least one position along said longitudinal axis; wherein a firstportion of said shaft is adapted to fixedly engage a first body, and asecond portion of said shaft is adapted to engage a second body and beretained with respect to said second body when said resilient member ispositioned around said fastener in a position adjacent to said at leastone position in which said resilient member is in a flexed state. 14) Asystem according to claim 13, wherein said first body is bone and saidsecond body is a surgically implantable device. 15) A system accordingto claim 13, wherein said resilient member is a split-ring spring thatsurrounds said shaft. 16) A bone screw system adapted to fasten asurgically implantable device to a bone, said bone screw comprising ahead having a proximal end of a first diameter, a middle portion of asecond diameter, and a distal end of a third diameter, said seconddiameter being greater than said first and third diameters; a shaftextending from the distal end of said head and being adapted to befixedly retained in said bone; a resilient retention member adapted tofit around said head in a manner in which said retention member isflexed when positioned around said second diameter, but said retentionmember is un-flexed when positioned around said third diameter; and areceiving opening in said surgically implantable device sized to preventremoval of said head and said retention member from therein when saidretention member is positioned around said first diameter. 17) A systemaccording to claim 16, wherein said head is threaded. 18) A systemaccording to claim 16, wherein said head is threaded over said middleportion. 19) A method of retaining a fastener, said method comprisingproviding a fastener having a first end and a second end, said first endbeing adapted to fixedly engage a first body, and said second end beingadapted to engage a second body; providing a resilient retention memberthat cooperates with said second body and said second end of saidfastener such that said fastener may be selectively changed between anon-retained condition and a retained condition, with respect to saidsecond body, when said retention member is changed between a relaxedstate and a flexed state, in order to join said first: body and saidsecond body; positioning said retention member in said second body;positioning said first body and said second body in close proximity toeach other; passing said fastener through said second body and into saidfirst body such that said fastener is retained in said first body, andmoving said retention member between a relaxed state and a flexed statein order to retain said fastener to said second body. 20) A methodaccording to claim 19 wherein said first end of said fastener comprisesscrew threads. 21) A method according to claim 19 wherein said firstbody comprises bone; and said second body comprises a surgicallyimplantable device. 22) A method according to claim 21 wherein saidfastener is a bone screw. 23) A method according to claim 21 whereinsaid surgically implantable device is adapted to span between at leasttwo bone structures. 24) A method according to claim 23 wherein saidsurgically implantable device is a plate that is fastened to each ofsaid at least two bone structures. 25) A method according to claim 19wherein said retention member is a split ring. 26) A method according toclaim 25 wherein said split ring has a first diameter when it is in arelaxed state, and is adapted to resiliently expand to larger diameterswhen it is in a flexed state; said second end of said fastener comprisesa locking portion of said fastener having a large diameter sectiongreater than said first diameter and positioned between two smallerdiameter sections; and said split ring is positioned within said secondbody such that said fastener is retained in said second body when saidsplit ring is positioned around one of said smaller diameter sections ina manner in which said large diameter section must be passed throughsaid split ring in order to remove said fastener. 27) A method accordingto. Claim 26, wherein said large diameter section is threaded. 28) Amethod for joining a first body to a second body, said method comprisingproviding a fastener comprising an elongated shaft of varying diameteralong a longitudinal axis; providing a resilient member adapted to beselectively positioned around said fastener and sized such that saidresilient member is in an un-stressed state at various positions alongsaid longitudinal axis, and said resilient member is in a stressed statein at least one position along said longitudinal axis; positioning saidresilient member in said second body; positioning a first portion ofsaid shaft through said second body and into said first body;positioning said resilient member around said fastener in a positionadjacent to said at least one position in which said resilient member isin a flexed state such that said shaft is retained with respect to saidsecond body. 29) A method according to claim 28, wherein said first bodyis bone and said second body is a surgically implantable device. 30) Amethod according to claim 28, wherein said resilient member is asplit-ring spring that surrounds said shaft. 31) A method of fastening asurgically implantable device to a bone, said method comprisingproviding a bone screw that has a head having a proximal end of a firstdiameter, a middle portion of a second diameter, and a distal end of athird diameter, said second diameter being greater than said first andthird diameters; and that has a shaft extending from the distal end ofsaid head and being adapted to be fixedly retained in said bone;providing a resilient retention member adapted to fit around said headin a manner in which said retention member is flexed when positionedaround said second diameter, but said retention member is un-flexed whenpositioned around said third diameter; positioning said retention memberinto a receiving opening in said surgically implantable device that issized to prevent removal of said head and said retention member fromtherein when said retention member is positioned around said firstdiameter; positioning said bone screw through said implantable deviceand through said retention member; and installing said bone screw intosaid bone. 32) A method according to claim 31, wherein said middleportion is threaded. 33) A plate system for orthopedic use, said systemcomprising a generally flat plate having a top surface, a bottomsurface, and at least one hole; a ring-shaped sleeve received in saidhole and having internal threads; a fastener adapted to be positionedthrough said hole and sleeve, and driven into a vertebra, said fastenerhaving a head portion with external threads adapted to mate with saidinternal threads; and a sleeve cover adapted to be attached to said flatplate top surface so that it overlaps said sleeve and prevents saidsleeve from being removed from said plate in a direction moving awayfrom the top surface of said plate. 34) A plate system according toclaim 33, wherein said sleeve cover comprises a generally flat memberhaving a screw-hole adapted to receive a screw therein for tighteningsaid sleeve cover to said plate. 35) A plate system according to claim33, wherein said hole is elongated. 36) A plate system according toclaim 35, further comprising a crescent-shaped spacer adapted to bereceived in said hole in order to occupy space within the hole adjacentto said sleeve when said sleeve is positioned in said hole. 37) A platesystem according to claim 35, further comprising an oval-shaped spacerhaving an opening therein and being adapted to be received in said holein order to occupy space within the hole when said sleeve is positionedin said opening. 38) A plate system for orthopedic use, said systemcomprising a generally flat plate having a top surface, a bottomsurface, and at least one hole; a ring-shaped sleeve received in saidhole and having at least one internal ridge; a fastener adapted to bepositioned through said hole and sleeve, and driven into a vertebra; aring locking member adapted to lock said fastener head relative to saidinternal ridge; and a sleeve cover adapted to be attached to said flatplate top surface so that it overlaps said sleeve and prevents saidsleeve from being removed from said plate in a direction moving awayfrom the top surface of said plate. 39) A plate system according toclaim 38, wherein said sleeve cover comprises a generally flat memberhaving a screw-hole adapted to receive a screw therein for tighteningsaid sleeve cover to said plate. 40) A plate system according to claim38, wherein said hole is elongated. 41) A plate system according toclaim 38, further comprising a crescent-shaped spacer adapted to bereceived in said hole in order to occupy space within the hole adjacentto said sleeve when said sleeve is positioned in said hole. 42) A platesystem according to claim 41, further comprising an oval-shaped spacerhaving an opening therein and being adapted to be received in said holein order to occupy space within the hole when said sleeve is positionedin said opening. 43) A surgically implantable system adapted to fastento a bone, said system comprising an implant device having at least onehole therethrough, said hole having a surface diameter at an uppersurface, an intermediate diameter greater than said surface diameter, asection of varying diameter between said surface diameter and saidintermediate diameter, and a lower surface positioned between said boneand said upper surface; a fastener positioned in said hole and haying ahead with a proximal end of a first diameter, a middle portion of asecond diameter, and a distal end of a third diameter, said seconddiameter being greater than said first and third diameters; a shaftextending through said hole from the distal end of said head and beingadapted to be fixedly retained in said bone; and a retention memberadapted to fit around said head in a manner in which said retentionmember is positioned between said surface diameter and said intermediatediameter. 44) A system according to claim 43, wherein said retentionmember is between and makes contact with said fastener head and saidsection of varying diameter in order to lock said fastener relative tosaid implant device. 45) A system according to claim 43, wherein saidretention member is resiliently expanded when positioned around saidfastener head. 46) A system according to claim 44, wherein said fastenerhead is adapted to be locked relative to said implant device in any oneof a variety of relative angular orientations. 47) A system according toclaim 44, wherein said fastener head is hemispherical. 48) A systemaccording to claim 43, wherein said retention member is ring-shaped. 49)A system according to claim 43, wherein said retention member is asplit-ring. 50) A system according to claim 48, wherein said retentionmember has an inner surface that varies in diameter in an axialdirection. 51) A system according to claim 48, wherein said retentionmember has an outer surface that varies in diameter in an axialdirection. 52) A system according to claim 49, wherein said retentionmember has an inner surface that varies in diameter in an axialdirection. 53) A system according to claim 49, wherein said retentionmember has an outer surface that varies in diameter in an axialdirection. 54) A system according to claim 48, wherein said retentionmember has an inner diameter that increases along an axial directionfrom top to bottom. 55) A system according to claim 48, wherein saidretention member has an outer diameter that increases along an axialdirection from top to bottom. 56) A system according to claim 49,wherein said retention member has an inner diameter that increases alongan axial direction from top to bottom. 57) A system according to claim49, wherein said retention member has an outer diameter that increasesalong an axial direction from top to bottom.